Flavor Formation By Enzymatic Reactions

Flavors associated with fats and oils sometimes originate through the activity of enzymes that first create fatty acid hydroperoxides and then volatile secondary products (16). The release of free fatty acids through lipolytic processes usually occurs before lipoxygenase, a major lipolytic enzyme, can catalyze the oxidation of the polyunsaturated fatty acids linoleic and linolenic acid to hydroperoxides. Lipoxygenase is limited to reactions with polyunsaturated fatty acids because of its substrate specificity for a cis,cis penta-l,4-diene unit with a methylene group at the co-8

position, a structure that is only present in fatty acids containing at least two nonconjugated double bonds. Therefore, the higher the polyunsaturated fatty acid content in a vegetable oil, the more susceptible the oil is to enzymatic oxidation and degradation.

Vegetable oils may also contain the enzyme hydroperoxide lyase, which is responsible for catalyzing the formation of aldehydes directly from the fatty acid hydroperoxides formed by lipoxygenase (17). The "grassy" or "beany" flavors often attributed to fatty acid autoxidation in vegetable oils may also originate from the formation of such volatile compounds as cis-3-hexenal and hexanal through the cleavage of hydroperoxides by hydroperoxide lyase.

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